AIAA-2000-4361
COST-BENEFIT STUDY OF FREE FLIGHT WITH
AIRBORNE SEPARATION ASSURANCE
Mario S.V. Valenti Clari*; Rob C.J. Ruigrok*; Jacco M. Hoekstra*
National Aerospace Laboratory NLR
P.O. Box 90502, 1006 BM, Amsterdam, the Netherlands
phone:+31 20 511 3012, fax.: +31 20 511 3210, e-mail: valenti@nlr.nl
NLR Free Flight Home Page
http://www.nlr.nl/public/hosted-sites/freeflight/
ABSTRACT
Free Flight has recently been proposed as a new
concept for a future Air Traffic Control (ATC)
system that can cope with the ongoing congestion of
the current traffic control system and, moreover, has
the potential to offer great economic benefits. The
research presented in this paper is based on the Free
Flight with Airborne Separation Assurance concept
that has been developed for the 1997 NASA Free
Flight project, conducted at the National Aerospace
Laboratory NLR in Amsterdam.
As a follow up of the 1997 project this paper
first focuses on the issues of fuel and time efficiency
of the conflict resolution manoeuvres in Free Flight
using the Airborne Separation Assurance concept as a
base line. An analysis is made of the fuel and time
efficiency of the possible conflict resolution
manoeuvres (heading change versus altitude change)
on a small scale by conducting so-called one-on-one
simulation experiments.
The next step is a cost-benefit analysis of
Free Flight on a large scale by simulating a mixedequipped traffic environment over a specified area in
European Airspace. These large-scale Monte-Carlolike simulation experiments have been set-up with
Free Flight (equipped) traffic flying direct routes and
non-equipped traffic flying along specified ATC
routes to their destinations. The analysis is aimed at
getting more insight in the costs and benefits of userpreferred routing combined with the airborne
separation assurance responsibility as is assumed in
the NLR concept for Free Flight.
INTRODUCTION
Due to the exponential growth of air traffic over the
last decades, the current Air Traffic Control (ATC)
system is reaching its flow capacity limits. The
removal of constraints upon traffic flow could allow a
more efficient user-preferred routing and the removal
of all constraints will eventually lead to realisation of
a Free Flight Air Traffic Management (ATM) system.
The radically different approach of Free Flight
gives the pilots/airlines the possibility to select their
routes freely. In return the pilots will be fully
responsible for the separation with other traffic,
instead of depending on a ground based Air Traffic
Controller (ATCo).
DEFINITION OF FREE FLIGHT
The general definition of Free Flight has been stated
in a concept paper called "Report of the Radio
Technical Commission for Aeronautics (RTCA)
Board of Directors' Select Committee on Free
Flight"1. The committee defines Free Flight as:
A safe and efficient flight operating capability under
Instrument Flight Rules (IFR) in which the operators
have the freedom to select their path and speed in
real time. Air traffic restrictions are only imposed to
ensure separation, to preclude exceeding airport
capacity, to prevent unauthorised flight through
special use airspace, and to ensure safety of flight.
Restrictions are limited in extent and duration to
correct the identified problem. Any activity which
removes restrictions represents a move toward free
flight.
The RTCA defines an air traffic system in which the
current ground-based (centralised) separation
assurance shifts to the cockpit; this implies an
(decentralised) airborne separation assurance concept.
The RTCA definition provides a base line
for a Free Flight concept with such an airborne
separation assurance system, nevertheless, the
* Master of Science Aeronautical Engineering (MSc)
Copyright © 2000 by the National Aerospace Laboratory NLR.
Published by American Institute of Aeronautics and Astronautics, Inc. with permission.
1
American Institute of Aeronautics and Astronautics
implications and, above all, the benefits to the users
(pilots, airlines) are still not quantified.
Preferred routes
FREE FLIGHT VERSUS PRESENT DAY ATC
Today’s airspace is organised in such way that allows
human air traffic controllers to detect and resolve
conflicts of traffic flying on predefined airways.
Figure 1a shows a schematic representation of this
current ATC situation.
a) ATC routes
b) Preferred routes
Figure 2 Airborne perspective
Figure 1 Schematic representation of an airway structure
versus user-preferred routing
By using airways (A,B,C and D) and also by
imposing altitude and speed restrictions to the
different airways a large amount of traffic can be
controlled by a single air traffic controller. However
these restrictions deny the aircraft to fly their optimal
route and moreover, a very small amount of the
available airspace is used.
If all restrictions would be removed in a way that
aircraft would be able to freely fly their preferred
(optimal) routes, the traffic pattern would more be
like shown in Figure 1b.
It is obvious that this situation is much more
complex to manage by a single controller, because
he/she has to identify for each aircraft the potential
conflicts. The controller then has to solve all
problems without creating new ones. Additionally
this has to be done within a short time span.
But if the situation is observed from the perspective
of a single aircraft, the picture becomes less
complicated; see Figure 2. When analysing the
picture one can see that out of the 18 aircraft only 4
aircraft might pose a problem in the near future (13,
11, 17 and 18).
In order to give the pilots the possibility to avoid the
conflicts, all aircraft must be properly equipped with
conflict detection and resolution tools. These tools
will detect all conflicts with the other aircraft in the
near future and provide the pilots with a resolution
advisory to solve these conflicts. So instead of
leaving the responsibility of separation to a
centralised ground-based traffic control the
responsibility shifts to the cockpit. This decentralised
concept of airborne separation assurance is the main
principle for NLR’s Free Flight studies.
Based on the concept of airborne separation
assurance, NLR has developed modules (algorithms)
that give pilots the capability of Conflict Detection
and Resolution (CD&R) for separation assurance.
The so-called Airborne Separation Assurance System
(ASAS) and related issues will be described in the
next section.
FREE FLIGHT WITH AIRBORNE
SEPARATION ASSURANCE
In 1997 the National Aerospace Laboratory NLR in
Amsterdam started a project to study the human
factors issues of Free Flight in co-operation with
NASA, the Federal Aviation Authority (FAA) and the
Dutch Aviation Authorities (RLD). The study
included off-line simulations to define a base-line
Free Flight concept, an Air Traffic Management
(ATM) safety analysis of the Free Flight concept and
a Human-in the-Loop simulation experiment to
investigate the impact of this new concept on human
factors. The studies resulted in the development of a
concept of Free Flight with Airborne Separation
Assurance.2,3
The main components of ASAS are the developed
conflict detection and resolution modules, which are
tools for a pilot to maintain separation with other
aircraft.
2
American Institute of Aeronautics and Astronautics
Conflict Detection
Conflict Resolution
A conflict is defined as a potential intrusion of a
protected zone in the near future. The task of the
conflict detection module is to predict such an
intrusion of the protected zone of the own aircraft by
other aircraft (intruders).
The protected zone is currently defined by
ATC standards as a circular zone of 5 nautical mile
radius and a height of 2000 ft (altitude - 1000ft to +
1000ft); see Figure 3.
For conflict resolution, the NLR ASAS uses the socalled Modified Voltage Potential (MVP) concept.
The modified voltage potential theory is based on
algorithms presented in a publication of the
Massachusetts Institute of Technology, Lincoln
Laboratory4. Figure 5 gives an illustration how the
concept works.
Figure 3 Protected zone (vertical scale exaggerated)
Figure 5 Modified voltage potential resolution method
The conflict detection module only detects conflicts
with aircraft for which the intrusion of the protected
zone takes place in the near future. The near future is
defined by using of a fixed look-ahead time of, for
example, five minutes. In this way an alert zone is
created, dependent of the aircraft’s airspeed and
direction of flight.
The conflict detection module uses the current state
(position and altitude) and trend vector (ground
speed, track and vertical speed) to detect conflicts.
Using vector calculations the predicted minimum
distance with other traffic is calculated. When less
than the required separation, and if time of intrusion
of the protected zone is within the look-ahead time,
the new conflict is detected. The conflict information
is presented to the crew on the CDTI displays
graphically. Figure 4 illustrates a Free Flight
Navigation display developed at NLR.
The conflict detection method is fail safe because
each future conflict is detected twice (by both
aircraft); meaning that a conflict will still be detected
when one of the conflict detection modules fails.
Both aircraft initially try to resolve the conflict
assuming the other will not manoeuvre. In practice
both aircraft do manoeuvre and that in general results
in fast conflict resolutions.
When the conflict detection module has predicted a
conflict with traffic, the resolution module uses the
predicted future position of the own aircraft
(ownship) and the obstacle aircraft (intruder) at the
moment of minimum distance.
The minimum distance vector is the vector
from the predicted position of the intruder to the
predicted position of the ownship.
The avoidance vector is calculated as the
vector starting at the future position of the ownship
and ending at the edge of the intruder’s protected
zone, in the direction of the minimum distance vector.
The length of the avoidance vector is the amount of
intrusion of the ownship in the intruder’s protected
zone and reflects the severity of the conflict. It is also
the “shortest way” out of the protected zone.
The ownship should try to accomplish this
displacement in the time left till the conflict; the lossof-separation time. Dividing the avoidance vector by
the time left yields a speed vector that should be
summed to the current speed vector. The result is an
advised track (heading change) and ground speed
(speed change). Using the three-dimensional vector
an advised vertical speed (altitude change) is
calculated also. In case of multiple conflicts within
the look-ahead time, the avoidance vectors are
summed.
FREE FLIGHT SIMULATION TOOL:
TRAFFIC MANAGER
The above-described concepts for conflict detection
and resolution have been tested using a tool for
simulating air traffic environments, called the Traffic
Manager (TMX).
Figure 4 Free Flight Navigation Display
With the TMX it is possible to generate a traffic
environment with various aircraft types. Both
automatic and interactively controlled traffic can be
generated by the TMX. For the simulation of the Free
3
American Institute of Aeronautics and Astronautics
Flight traffic the TMX uses six-degrees-of-freedom
models containing auto-pilot and auto-throttle
functionality, flight functionality and a pilot model.
The pilot model includes a delayed reaction
to conflict resolution advisories and a delayed
resuming of navigation to the aircraft’s destination,
once a conflict is solved. The resolution advisories
from the conflict detection and resolution algorithms
are taken over by the pilot models, thus controlling
the auto-pilot to resolve the conflict.
The TMX formed the simulation tool for the research
presented in this paper. In Figure 6 a screen shot of
the Traffic Manager is shown.
Figure 6 Traffic Manager screen layout
ISSUES OF THE 1997 HUMAN-IN-THE-LOOP
EXPERIMENTS
In the previous paragraphs the main components of
ASAS have briefly been described. The system has
been extensively tested for Human Machine Interface
(HMI) and Human Factors (workload, safety, and
acceptability) issues during NLR’s 1997
experiments5.
The overall conclusion of the studies was
that the feasibility of Free Flight with Airborne
Separation Assurance could not be refuted if all
aircraft were fully equipped with conflict detection
and resolution tools. Nevertheless, as with all
research, the experiments also raised some key
questions.
For example, it was noticed during the experiments
that pilots preferred to resolve conflicts by
manoeuvring horizontally; meaning they preferred
executing a heading change above executing an
altitude or speed change to resolve conflicts with
other aircraft.
This is somewhat strange because, when using
heading in order to resolve a conflict, the aircraft will
often need to manoeuvre more than when using an
altitude (vertical speed) change. It should be kept in
mind that the protected zone can be observed as a
very flat disc (the width-height ratio is similar to a
coin) flying through space. This implies that in a
conflict situation the amount of horizontal intrusion
(maximum 5 nm) will often be of a much greater
order than the vertical intrusion (maximum 1000 ft).
In the experiment debriefings, pilots explained that
they avoided vertical manoeuvres because they
thought it would have a negative impact on both,


the fuel efficiency of the flight (economic
aspects)
the passengers perception of the ride quality
(passenger comfort aspects)
The option of using speed changes for conflict
resolution was used even more seldom, because pilots
thought that the available (operational) speed window
in cruise flight would not allow this kind of conflict
resolutions.
An extra study was needed to give more insight
in the costs and benefits of the conflict resolution
manoeuvres (heading change, altitude change and
speed change) in Free Flight with Airborne
Separation Assurance.
In order to give more insight in the issues raised in
the Human-in-the-Loop experiments, NLR started in
1998 a preliminary cost-benefit study of Free Flight
with Airborne Separation Assurance. The work
presented in this paper was performed by the author
as part of a graduation assignment for the Faculty of
Aerospace Engineering, Delft University of
Technology (TUD).6
The benefit study can be divided into two
major parts. The first part deals with a study of the
costs and benefits of the conflict resolution
manoeuvres on a small scale. The second part zooms
out, in order to compare a full scale Free Flight
environment with an ATC environment like today.
This large-scale analysis is aimed at getting more
insight in user-preferred routing costs and benefits of
the airborne separation assurance concept.
COST-BENEFIT STUDY OF CONFLICT
RESOLUTION MANOEUVRES IN
FREE FLIGHT
As a first step in understanding the economic aspects
of conflict resolutions manoeuvring, several one-onone conflicts were tested on fuel and time efficiency.
The one-on-one conflicts were simulated with
horizontal (heading change) and vertical resolutions
(altitude change) in such a way that results could be
compared. This chapter deals with the set-up, results
and issues of these experiments.
EXPERIMENT SET-UP
The aim of the experiments was to compare the
horizontal conflict resolution (heading change only)
with the vertical conflict resolution in several one-onone conflicts. The method used for the experiments is
based on the idea of choosing the position of a large
4
American Institute of Aeronautics and Astronautics
number of experiment points in the protected zone of
an intruder aircraft. Each experiment point represents
a minimum distance point for a conflict that will
occur during an experiment.
The minimum distance point is the most
important parameter for the conflict resolution
module because it indicates the amount of intrusion
(0 – 5 nm). The experiment points for the horizontal
and vertical conflict experiments have been chosen as
shown in Figure 7.
point of minimum distance is located at a desired
experiment point in the protected zone of the intruder.
For this purpose the horizontal experiments
have been arranged in four initial experiment
situations. All experiment situations are related to the
position of the predefined points in the protected zone
of the intruder aircraft. The points are chosen on four
lines (a,b,c and d) as illustrated in Figure 9 below.
Direction of Flight
Intruder
b) Side View
Vertical Experiment Points
a) Top View
Horizontal Experiment Points
situation line a
d
1000 ft
200 ft
b
d
5 nm
1 nm
b
a
5 nm
Figure 7 Predefined experiment points in horizontal and
vertical plane (scales exaggerated)
The points are defined for various amounts of
intrusion with an interval of 200 ft. For the horizontal
resolutions the amount of horizontal intrusion is
chosen with a 1 nm interval. The experiments have
been subdivided like this because for the vertical
resolution method only the amount of vertical
intrusion will be important and for the horizontal
resolution method only the horizontal amount of
intrusion. This subdivision makes the task of
comparing the two resolution methods much easier.
Figure 9 Situation lines for horizontal experiments
The experiment points on, for example, lines b and d
are related to the initial experiment situation b and
situation d as illustrated in Figure 10.
protected zone intruder (b)
reference flight track ownship
destination
initial position ownship
(lat : 0.00 lon : 2.00)
(lat :0.00 lon : 0.00)
initial position intruder
situation d
flight path intruder (b)
Horizontal Conflict Experiments
The general experiment set-up has been chosen as
follows. Each experiment starts with two aircraft
flying with constant speeds and altitudes according a
predefined scenario. One of the two aircraft will be
observed as experiment aircraft (own aircraft) the
other is the intruder; see for example Figure 8.
initial position intruder
experiment area
experiment area
flight path intruder (d)
initial position intruder
situation b
Figure 10 Example of situation b and situation d
experiments
When a conflict is detected the own aircraft will
manoeuvre in order to resolve the conflict. The
intruder will hold his track without manoeuvring; so
the own aircraft will completely have to resolve the
conflict (non-nominal case). When the conflict is
resolved the aircraft will hold its flight track for a
predefined time interval until it is time to direct back
to the destination; Figure 11.
protected zone intruder
at min dist.
initial position intruder
ownship turning to destination
initial position own
(lat :0.00 lon : 0.00)
min dist
reference flight track own
destination
(lat : 0.00 lon : 2.00)
direct route to destination
destination
flight track intruder
start of conflict
resolution manoeuvre
reference flight track own
Figure 8 Example experiment situation
flight track intruder
The flight path of the own aircraft is a direct flight
over 120 nm to a destination on the edge of the
experiment area. Each experiment stops when the
experiment aircraft exits the experiment area. The
initial position of the intruder is chosen in a way that
when a conflict is detected during flight, the initial
Figure 11 After resolving the conflict with the ownship
follows a direct route to the destination
5
American Institute of Aeronautics and Astronautics
- 11 climbs at FL200 & FL300
88 horizontal resolution experiments
(4 situations of 11 points at FL200 & FL300)
2 reference flights
(without manoeuvring)
Vertical Conflict Experiments


Vertical Conflict Experiments
The results for the climb manoeuvres at FL300 are
presented in Figure 13 and Figure 14.
Vertical Resolution Method
Climb to Resolute Conflict
32500
Fokker 100
W = 38000 kg
M = 0.70
ISA conditions
no wind
32000
altitude difference: +1000 ft (c999)
altitude difference: +800 ft (c800)
altitude difference: +600 ft (c600)
altitude difference: +400 ft (c400)
altitude difference: +200 ft (c200)
altitude difference: 0 ft
altitude difference: -200 ft (c_200)
altitude difference: -400 ft (c_400)
altitude difference: -600 ft (c_600)
altitude difference: -800 ft (c_800)
altitude difference: -1000 ft (c_999)
31500
Altitude [ft]
Vertical conflict experiments have been executed
with a similar set-up as the horizontal conflict
experiments. In all the tests, the intruder aircraft was
on a head-on collision course with the own, because
only the vertical amount of intrusion needed to be
varied.
Nevertheless, only the ASAS conflict
detection module (not the resolution module) has
been used for the execution of the vertical
experiments; a standard altitude change procedure has
been used for the vertical conflict resolutions. The
reason for this approach is to focus on the efficiency
of manoeuvres, as pilots would execute in Free Flight
with airborne separation assurance. For the relevance
of the study it was decided to implement a procedural
approach of resolving the vertical resolution
manoeuvres in which the ownship has been assigned
to resolve all conflicts with:
31000
30500
30000



a climb/descent with constant Mach number
a level-off altitude of 100ft above/below the
intruder aircraft’s protected zone
a fixed vertical speed of 600ft/min
29500
0
20
40
60
80
100
120
Ground distance [nm]
Figure 13 Flight paths for Climb manoeuvres at FL300
Vertical Resolution Method (Climb)
Comparison of Fuel and Time Efficiency with reference flight
The vertical manoeuvre is illustrated in Figure 12.
M = 0.70
ISA conditions
no wind
Reference altitude: 30,000 ft (FL300)
+999 ft
+800 ft
100 ft
climb
600 ft/min
1000 ft
min.dist
intruder
1000 ft
Experiment point [-]
+600 ft
protected zone
intruder
+400 ft
+200 ft
-400 ft
reference flight track
ownship
ownship
descent
600 ft/min
Flight time
Fuel consumed
0 ft
-200 ft
-600 ft
-800 ft
100 ft
-999 ft
-5.00
-4.00
-3.00
-2.00
Figure 12 Vertical conflict resolution manoeuvre
The ownship is assigned to return to the original
altitude after waiting a predefined time interval when
the conflict has been resolved.

44 vertical resolution experiments
- 11 descents at FL200 & FL300
3.00
4.00
5.00
Horizontal Conflict Experiments
The results for the situation a at FL300 are presented
in Figure 15 and Figure 17
Horizontal Resolution Method (Situation a)
Flight Tracks
20
Fokker 100
W = 38000 kg
M = 0.70
ISA conditions
no wind
15
30000 ft
10
distance [nm]
5
0
0
20
40
60
-10
The complete experiment matrix of the one-on-one
experiments consisted of
2.00
Figure 14 Fuel burned and time used compared to the
reference flight at FL300
-5
RESULTS
1.00
Compared to reference [%]
Aircraft performance validation
All aircraft models simulated in the TMX are based
on BADA (Base of Aircraft Data) aircraft
performance data7. All results presented in this paper
have been generated with a medium range twinengine aircraft of the TMX. The performance of the
used BADA aircraft model has been validated by
comparing it with the much more sophisticated
simulation model of the same aircraft used in NLR’s
Research Flight Simulator (RFS).
0
-1.00
80
100
120
intrusion: 0 nm (a050)
intrusion: 1 nm (a040)
intrusion: 2 nm (a030)
intrusion: 3 nm (a020)
intrusion: 4 nm (a010)
intrusion: 5 nm (a000)
intrusion: 4 nm (a_10)
intrusion: 3 nm (a_20)
intrusion: 2 nm (a_30)
intrusion: 1 nm (a_40)
intrusion: 0 nm (a_50)
-15
-20
distance [nm]
Figure 15 Flight tracks for situation a at FL300
6
American Institute of Aeronautics and Astronautics
Horizontal Resolution Method (Situation a)
Comparison with Reference
situation a +5nm
M = 0.70
ISA conditions
no wind
30,000 ft (FL300)
situation a +4nm
Experiment points [-]
situation a +3nm
situation a +2nm
situation a +1 nm
situation a +0 nm
9.4%
Flight Time
Fuel Consumed
situation a –1 nm
situation a –2 nm
situation a –3 nm
situation a –4 nm
situation a –5 nm
-5.00
-4.00
-3.00
-2.00
-1.00
0
1.00
2.00
3.00
4.00
5.00
Compared to Reference [%]
Figure 16 Fuel burned and time used compared to
reference flight (situation a)
DISCUSSION
When analysing the results it should be clear that all
experiments were based on some constraining
assumptions that implicate a certain level of
simplification. The aim of the experiments is to get a
better understanding of the economic aspects of the
resolution methods.
One of the most determining factors is expected to be
the type of aircraft because optimal flight issues are
very dependent of the type of aircraft. The
experiments have been executed with a simulation
model that estimates the behaviour of a medium
range twin-engine civil aircraft. Another factor that
could influence the performance is the environmental
condition (e.g. wind).
When analysing and comparing the fuel consumption
of all experiments it is clear that in one case the
experiment aircraft saves fuel with respect to the
reference flight over the defined trajectory. This
occurs when the aircraft performs a vertical climb to
resolve the conflict; see Figure 13 and Figure 14.
Figure 14 shows that for all experiment points
(different intrusions in the protected zone of the
intruder) the total fuel consumed is less than the
reference value. The low points in the protected zone
show the biggest gain. This makes sense because, for
these low intrusions, the experiment aircraft has to
perform a high altitude step in order to resolve the
conflict; bringing it to a more optimal cruise level.
This implicates that after performing the altitude step
it would maybe be even more efficient to remain at
the higher level. Of course the distance to destination
also influences this decision.
When assuming a constant Mach number
(and flight in the troposphere), the true airspeed (and
also the ground speed) will decrease with the
increasing altitude. This means that the aircraft will
arrive later on its destination, which can also be read
from Figure 14. The amount of time lost is however
very small; in the order of a few seconds for the
experiment flight over a distance of 120nm.
The results from the vertical climb resolution are very
promising when regarding the fuel consumption
figures. However, there are some issues that could
seriously constrain this resolution manoeuvre. It is
likely to assume that pilots, when they are given the
user-preferred routing possibility, will perform the
cruise flight as close as possible to the operational
ceiling of the aircraft; especially on the long routes.
When the pilot wants to perform a climb in order to
resolve a conflict it could well be possible that this is
constrained by the ceiling. Other aspects, like the
influence of the engine spool-up noises (e.g. when
performing climbs near the operational ceiling) on the
passenger comfort, could also pose a constraint on the
climb manoeuvre.
So, assuming for the moment that the climb
manoeuvre is often not an option, this leaves the
vertical descent manoeuvre and the horizontal
heading change as the possibilities to resolve the
conflict. A trade-off can be found between the
advantages and disadvantages of all manoeuvres.
This trade-off between the horizontal
heading change manoeuvre and the vertical descent
manoeuvre can be combined with the MPV concept
for the set-up of a decision model. Figure 17
illustrates the decision with the protected zone of the
intruder subdivided in zones for different resolution
options.
r
r1
1 = 1.5 nm
Climb
Heading
Heading
Descent
r2 = 4 nm
r2
R = 5 nm
R
r2
R
r1
Figure 17 Decision model for resolution method
If the initial position of the minimum distance point is
located in the upper half of the protected zone, the
vertical climb resolution is the most optimal
manoeuvre. The figure illustrates in the lower half of
the protected zone the trade-off between the
horizontal (heading change manoeuvre) and the
vertical descent manoeuvre. The vertical climb
manoeuvre is in the lower half of the protected zone
not an option because it would go against the MVP
concept.
It can be concluded that the use of the vertical
resolution method is not as bad for the fuel
consumption as thought by some of the pilots who
participated in the 1997 Human-in-the-Loop
7
American Institute of Aeronautics and Astronautics
experiments. The vertical climb manoeuvre could
even lead to a more efficient flight operation.
However, if the climb manoeuvre is not possible the
geometry of the conflict (the position of the minimum
distance points in the protected zone) can be used to
determine what is better: a descent manoeuvre or a
heading change.
The next chapter will try to analyse a more general
Free Flight environment in which aircraft fly their
routes from airport to airport.
MONTE-CARLO FREE FLIGHT
EXPERIMENTS WITH MIXED-EQUIPPED
TRAFFIC
This chapter will present the set-up, issues and results
from Monte-Carlo like simulations of a full-scale
Free Flight traffic environment. The simulations have
been executed in an area of European airspace with
ASAS equipped traffic flying in the same area as notequipped traffic, flying along specified ATC routes
from airport to airport. The ultimate goal of the
experiments was to find out if the “benefits” of userpreferred routing outweighs the “costs” of conflict
resolution manoeuvres related to the airborne
separation assurance concept.
MIXED EQUIPPED TRAFFIC CONTROL
ENVIRONMENT
Free Flight has been proposed primarily for future
application because it has the potential to cope with
the ongoing congestion of the current ATC system.
Besides the foreseen increase in airspace capacity,
Free Flight could also offer great economic
advantages by eliminating the costs related with the
fuel wasted when flying on non-direct ATC routes.
On the other hand, the Airborne Separation
Assurance concept implies that aircraft will be
responsible for their own separation assurance. It is
not yet clear to what extent the occurrence of
conflicts will influence the fuel consumption on a
global level. It has been suggested that the advantage
of user-preferred routing in Free Flight is eliminated
when a great number of conflicts have to be resolved.
The Human-in-the-Loop experiments showed that the
number of conflicts that occurred in a real-flight
scenario, in an above-nominal traffic density, was not
very high5. These experiments were executed in a full
Free Flight environment (all aircraft equipped with
ASAS). The question remains if the number of
conflicts will remain low in a mixed-equipped traffic
environment.
In order to compare Free Flight (direct
routing traffic) with current ATC, the Monte-Carlo
Free Flight experiments simulate “normal” ATC
traffic flying specified routes from airport to airport;
flying in the same airspace as the Free Flight traffic.
This resulted in a so-called “mixed-equipped” traffic
environment. In this environment the traffic equipped
with ASAS is not only responsible for separation
assurance with the other Free Flight traffic, but will
also have to avoid the “normal” traffic flying on the
ATC routes.
The conflict resolutions during the
experiments were executed using NLR’s Airborne
Separation Assurance concept. However, the decision
between the horizontal and the vertical manoeuvre
was made by a dedicated decision module, which was
developed, based on the results and experiences from
the one-on-one experiments; see previous chapter.
EXPERIMENT SET-UP FOR MONTE-CARLO
LIKE SIMULATIONS
In order to simulate (using the TMX) a “realistic”
mixed-equipped traffic environment, the experiments
have been set-up over a specified area in (virtual)
European airspace. An experiment area (440 nm x
360 nm) was set-up that included the following four
European airports:
1.
2.
3.
4.
Amsterdam Airport Schiphol in the Netherlands
(EHAM)
Frankfurt/Main Airport in Germany (EDDF)
Paris Charles-de-Gaulle Airport in France
(LFPG)
London Heathrow Airport in England (EGLL)
Twelve ATC routes have been defined connecting
these experiment airports, using official Jeppesen
European route charts and the navigation database.
The routes were defined from Terminal Area (TMA)
to TMA, because the terminal area manoeuvrings
were not included in the experiments.
Figure 18 Screenshot of the TMX with the experiment
area and ATC routes
Six extra airports outside the experiment area and one
airport in the experiment area were defined for a
realistic mix of high cruising, departing and arriving
traffic in the experiment area.
8
American Institute of Aeronautics and Astronautics
All simulations were executed using the automatic
traffic scenario generation functionality of the TMX,
which was also used to generate scenarios for the
Human-in-the-Loop experiments. Using this
functionality the TMX can be assigned to constantly
generate traffic departing from airports and en-route
entering-points (traffic sources).
When an aircraft is generated at an airport it
will enter the traffic environment at a predetermined
point on the edge of the TMA. The location of this
point depends on the direction of the destination with
respect to the origin. The altitude of departure can be
specified. The destinations of the aircraft are
randomly selected from a user-determined list of
airports.
In order to obtain a realistic mixed-equipped
traffic environment the generation of aircraft during
the experiments was split in the generation of Free
Flight traffic (equipped with ASAS) and the
generation of non-equipped ATC traffic, flying the
defined routes from origin-TMA to destination-TMA.
The selected experiment airports (EHAM,
EGLL, EDDF and LFPG) generated a specified
percentage of ASAS equipped aircraft. All other
airports only generated Free Flight traffic. This
implied that the traffic environment mainly consisted
of Free Flight traffic. The mix was only applied to the
direct/ATC routes between the experiment airports.
percentage) with approximately a 2-minute take-off
interval on the mentioned airports. The aircraft
departing from the experiment airports all used the
same aircraft model. These aircraft were all instructed
to fly to a destination within the experiment area
along a direct route or along the defined ATC route.
Aircraft that were generated at the other airports were
instructed to fly Free Flight to all other airports (also
the airports specified in the experiment area).
The experiment aircraft that were generated at the
five specified airports in the experiment area were
used for the measurement of relevant parameters.
Each time when such an aircraft reached its
destination, it was deleted from the simulation and a
set of parameters was sampled (e.g. fuel consumed,
flight time).
In order to obtain a representative measurement the
sampling of parameters was not started until the
number of aircraft present in the experiment area
stabilised. Each time an experiment was started the
number of aircraft flying in the area increased over
the first hours of simulation. This build-up of aircraft
slowly decreased after a few hours of simulation, as
illustrated in Figure 19.
Number of Aircraft
during Mixed Equipped Traffic Simulations
350
EXPERIMENT MATRIX
The experiments have been executed for five
different levels of equipment percentage on the
routes:
2.
3.
4.
5.
0% equipped with ASAS; 100% not-equipped;
complete ATC environment
25% equipped with ASAS; 75% not-equipped;
50% equipped with ASAS; 50% not-equipped;
75% equipped with ASAS; 25% not-equipped;
100% equipped with ASAS; 0% not-equipped;
complete Free Flight environment
EXECUTION OF SIMULATION
EXPERIMENTS; AUTOMATIC SCENARIOS
Within the scenario, the TMX constantly generated
traffic (according to the desired equipment
Number of Aircraft [-]
All the simulation experiments have been executed
under assumption that a ground ATC ideally
controlled the non-equipped traffic. When conflicts
(and intrusions) did occur, they were ignored.
When analysing the results this must be kept
in mind because it implies that the fuel consumption
of the ATC traffic will be too optimistic; in a real
flight situation ATC traffic will rarely fly routes
without extra manoeuvring or restrictions for the
avoidance of conflicts.
1.
Sampling started
after 9550 seconds
300
250
200
example
150
100
50
0
0
5000
10000
15000
20000
25000
Time [s]
Figure 19 Example of aircraft build-up in the simulated
traffic environment
Each experiment (for every percentage of equipment)
lasted five hours and was repeated four times. This
means that for every equipment percentage a total of
twenty hours of sampling was reserved.
RESULTS
The data that resulted from the experiments has been
gathered and processed in order to obtain a relevant
insight in how the direct-routing advantage of the
Free Flight aircraft is compared to the aircraft flying
ATC (conflict free) routes. The most important
results will be shown in this paragraph.
Figure 20 illustrates the total flight fuel
consumption averaged over all flights (and all routes).
It gives a very global comparison of the efficiency of
the simulated Free Flight traffic with the simulated
ATC traffic.
9
American Institute of Aeronautics and Astronautics
Monte-Carlo Simulations of a mixed equipped traffic environment
Total Fuel Consumed averaged over all flights
Monte-Carlo Simulations of a mixed equipped traffic environment
Total Number of Conflicts averaged over all flights
9
1600
Averaged Standard Deviation
Averaged Standard Deviation
Fuel consumed [kg]
1400
1200
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
FF traffic test 1
FF traffic test 2
FF traffic test 3
FF traffic test 4
1000
800
600
400
6
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
FF traffic test 1
5
4
FF traffic test 2
FF traffic test 3
FF traffic test 4
3
1
0
0
0
25
50
75
100
0
Percentage of equipped traffic [%]
The figure shows the averages of all the experiments
in a way that the results of various test-runs can be
compared easily. Moreover, in order to give some
insight in the scatter of the samples, the averaged
standard deviation is shown for the second
experiment series.
Figure 24 shows the global results for the total flight
distance and Figure 22 for the total flight time.
Monte-Carlo Simulations of a mixed equipped traffic environment
Total Flight Distance averaged over all flights
300
Averaged Standard Deviation
250
200
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
FF traffic test 1
150
FF traffic test 2
FF traffic test 3
FF traffic test 4
100
50
0
0
25
50
75
100
Percentage of equipped traffic [%]
Figure 21 Global view of the total flight distance for Free
Flight and ATC traffic
Monte-Carlo Simulations of a mixed equipped traffic environment
Total Flight Time averaged over all flights
Averaged Standard Deviation
2500
2000
1500
ATC traffic test 1
ATC traffic test 2
ATC traffic test 3
ATC traffic test 4
FF traffic test 1
1000
FF traffic test 2
FF traffic test 3
FF traffic test 4
500
0
0
25
50
75
25
50
75
100
Percentage of equipped traffic [%]
Figure 20 Global view of the total fuel consumption for
Free Flight and ATC traffic
Distance [nm]
7
2
200
Time [s]
Number of Conflicts [-]
8
100
Percentage of equipped traffic [%]
Figure 22 Global view of the total flight time for Free
Flight and ATC traffic
Finally Figure 23 shows the average number of
conflicts encountered during flight for ATC and Free
Flight traffic.
Figure 23 Global view of the number of conflicts for Free
Flight and ATC traffic
DISCUSSION
The goal of the full-scale Monte Carlo like
simulations was to make a first estimation of the
global performance of Free Flight traffic (flying
along direct routes) compared to normal ATC traffic
(flying along predefined routes, free of conflicts). The
role of a mixed-equipped traffic environment was
also an issue of investigation.
The experiment set-up was somewhat in the
advantage for the traffic flying along the specified
ATC routes. There were no delays on the routes and
the conflicts with other traffic were neglected,
assuming an ideal traffic flow management. This
leads on the forehand to the assumption that there
would be no great differences between the
performance of the Free Flight traffic and the ATC
traffic.
In spite of this, the results of the experiments
were still in favour of Free Flight traffic. Figure 20
shows that the overall fuel consumption of the Free
Flight aircraft, averaged over all flights, is
significantly lower than the overall fuel consumption
of the ATC traffic. The results of the separate tests
are all in the same range indicating the reliability of
the test results. The increase in equipment percentage
shows a slight decrease of overall fuel consumption.
This can be explained as follows. With an
increasing percentage of equipment increases the
chance that when a conflict occurs, it will be between
equipped aircraft, because there are less not-equipped
aircraft in the traffic environment. This implies that
the aircraft can use smaller manoeuvres to resolve
conflicts, leading to lower fuel consumption.
Figure 23 shows that the number of conflicts that
occur during flight is very diverse. The averages are
about the same value for all the tests, however, the
average standard deviation shows that the
measurements are extremely wide spread. When
analysing the data it is found that some of the flights
experience a lot of conflicts and other have
practically no conflicts at all. The extremes go to over
thirty conflicts on some occasions.
10
American Institute of Aeronautics and Astronautics
The explanation of this phenomenon is that in some
occasions Free Flight aircraft get stuck in a conflict
situation involving more aircraft. An example of such
a situation is illustrated schematically in Figure 24.


Conflict
Ownship
Intruder 1
Intruder 2

Imminent
Conflict
It is refuted that the vertical resolution
manoeuvre always has a negative impact on fuel
efficiency.
The vertical climb manoeuvre is a very fuelefficient way of resolving conflicts.
A trade-off has been found between the vertical
descent manoeuvre and the heading change
manoeuvre with respect to fuel consumption and
time.
ATC route
Figure 24 Example of a problem conflict situation
The ownship is flying a direct route to its destination
when it suddenly detects a conflict with another
equipped aircraft (intruder 1). The intrusion of the
conflict is small and the decision model induces a
heading change manoeuvre. The manoeuvre resolves
the conflict with intruder 1 but causes a new conflict
with another aircraft (intruder 2) flying along an ATC
route. Again, the decision model induces a heading
change, however, now in the opposite direction. This
causes the reoccurrence of the old conflict (but
counted as a new conflict) with intruder 1. This
process can be repeated until one of the equipped
aircraft decides to resolve the conflict with an altitude
change.
The situation occurs because of the
simplicity of the decision model in the resolution
module. A human pilot, with the help of proper
CD&R tools and CDTI, could have resolved this
situation easily. The described problem situation
occurs several times in all the experiments. In spite of
this, the average of the results is hardly influenced by
the peak values; only the standard deviation indicates
its presence.
It can be concluded that a decision model
solemnly based on one-on-one conflict encounters is
not enough for efficient Free Flight. When resolving
a conflict it should first be determined if the
resolution causes new conflicts (or worse: old
conflicts).
CONCLUSION
The research presented in this paper was aimed to
make a first inquiry into the issues raised in the 1997
Human-in-the-Loop study. This has been done by
first observing Free Flight on a very small scale, by
conducting one-on-one conflict experiments. Using
the knowledge of these experiments, large-scale tests
have been conducted with Free Flight aircraft flying
in a “mixed-equipped” traffic environment.
With respect to the small-scale one-on-one
experiments the following conclusions can be made:
With respect to the large-scale Monte Carlo
simulation experiments the following conclusions can
be made:





The direct-routing benefits of Free Flight
outweigh the costs related to the Airborne
Separation Assurance concept.
A simple decision model has been developed that
combines the results of the small-scale
experiments with the MVP concept.
In some complex conflict situations (often
involving mixed-equipped traffic) the decision
model still lacks the human-like anticipation for
a more efficient (fast) resolution of conflicts.
The average number of conflicts encountered by
Free Flight aircraft in the mixed-equipped
environment is very low, nevertheless, the results
are widely scattered, which can be blamed on the
simplicity of the decision model
The MVP algorithms have proven to work in a
mixed-equipped traffic environment.
Finally, the results of the Monte-Carlo study were
beyond expectations. It can be concluded that the
direct-routing aspects of Free Flight, indeed gives it
great potential to reduce the fuel-related costs in the
traffic environment. This conclusion combined with
the conclusion of the preceding NLR Free Flight
studies, encourage further studies before introduction
of Free Flight in a future traffic control environment.
REMAINING ISSUES:
CURRENT AND FUTURE STUDIES
A remaining issue with respect to the study presented
in this paper is the issue of competitiveness between
users. In what extent does competitiveness between
users effect the cost-benefit aspects of airborne
separation assurance.
The results presented in this paper were all
based on experiments without actual human
interaction; all conflict resolutions were executed by
pilot (decision) models flying simulated aircraft. A
critical objection to the results could be that in a real
life situation pilots would not be so anxious to resolve
all occurring conflicts, but would anticipate the
manoeuvres of the other aircraft involved.
11
American Institute of Aeronautics and Astronautics
Another issue that remains is the effect of the
performance of the system providing the traffic
information to the users of the ASAS CD&R. Free
Flight with Airborne Separation Assurance assumes
that traffic information is available via a datalink
system such as ADS-B. Nevertheless, the
performance of the used system could have impact on
several aspects of the system. An obvious field of
interest with respect to ADS performance issue will
be human factors: what is the minimum required (and
preferred) update rate of the CDTI displays in the
Free Flight cockpit.
Currently the NLR studies are focussing on the
above-mentioned issues. In the spring and summer of
2000 NLR (under contract by NASA, FAA and RLD)
will execute the so-called Human Interaction
Experiments (HIE) in which “pilots” from all over the
world can participate in Free Flight simulation
experiments via the Internet. The earlier described
TMX will be used as Free Flight simulation
environment that will be linked to a great number of
desktop Free Flight aircraft simulation applications
(FreeSim) using the globally-used TCP/IP protocol;
an illustration is given in Figure 25
[3]
[4]
[5]
[6]
[7]
Assurance”, Proceedings of the Confederation
of European Aerospace Societies (CEAS) 10th
European Aerospace Conference, 1997,
Amsterdam
Gent, R.N.H.W. van, Hoekstra, J.M., Ruigrok,
R.C.J., “Conceptual Design of Free Flight
with Airborne Separation Assurance”, AIAA98-4239, Guidance, Navigation and Control
Conference, 1998, Boston
Eby, M.S., “A Self-Organizational Approach
for Resolving Air Traffic Conflicts”, The
Lincoln Laboratory Journal Vol.7, Nr.2, 1994
Hoekstra, J.M., Gent, R.N.H.W. van, Ruigrok,
R.C.J., “Man-in-the-Loop Part of a Study
Looking at a Free Flight Concept”, Digital
Avionics System Conference, Seattle, 1998
Valenti Clari, M.S.V., “Cost-Benefit Study of
Conflict Resolution Manoeuvres in Free
Flight”, Delft University of Technology,
Graduation Report, Delft, 1998
Bos, A., “User Manual for the Base of
Aircraft Data (BADA) Revision 2.6”, EEC
Note 23/97, 1997
ABBREVIATIONS & ACRONYMS
NLR Human Interaction Experiment
ADS-B
Free Flight on the Internet
ASAS
ATC
ATCo
ATM
CDTI
TCP/IP
FreeSim
Player
TMX Server
Figure 25 Human Interaction Experiment
Future work of NLR will be focussed on expanding
the application of the state-based Free Flight concept
in the aircraft operational envelope. The ASAS will
be tested in a more complete flight envelope from
TMA to TMA. Future experiments will include
transition from Free Flight Airspace to Managed
Airspace and vice versa; the goals for the near future
is expanding the knowledge of application of Free
Flight with Airborne Separation Assurance in the
complete operational envelope.
FAA
FreeSim
HIE
HMI
IFR
IP
MVP
NASA
NLR
RFS
RLD
RTCA
REFERENCES
[1] RTCA, “Report of the Radio Technical
[2]
Commission for Aeronautics (RTCA) Board of
Directors’ Select Committee on Free Flight”
Gent, R.N.H.W. van, Hoekstra, J.M., Ruigrok,
R.C.J., “Free Flight with Airborne Separation
TCP
TMA
TMX
TUD
Automatic Dependent Surveillance
Broadcast
Airborne Separation Assurance
System
Air Traffic Control
Air Traffic Controller
Air Traffic Management
Cockpit Display Traffic
Information
Federal Aviation Authority
Free Flight Desktop Simulation
Human Interaction Experiment
Human Machine Interface
Instrument Flight Rules
Internet Protocol
Modified Voltage Potential
National Aeronautics and Space
Administration
Nationaal Lucht- en Ruimtevaartlaboratorium (National Aerospace
Laboratory)
Research Flight Simulator
Rijks Luchtvaart Dienst (Dutch
Aviation Authority)
Radio Technical Commission for
Aeronautics
Transmission Control Protocol
Terminal Manoeuvring Area
Traffic Manager
Technische Universiteit Delft (Delft
University of Technology)
12
American Institute of Aeronautics and Astronautics